This invention relates generally to injection molding and, more particularly, to an apparatus and method for controlling gate drool.
It is well known in thermally-gated hot runner injection molding systems that runner decompression prior to, or contemporaneous with, parting the mold advantageously reduces gate melt drool. Gate drool adversely affects the quality of succeeding parts and should therefore be avoided.
In typical injection molding systems, it has been found that decompressing the molding machine greater auger prior to, or upon, parting the mold advantageously reduces gate drool. Decompression decreases the pressure within the hot runner in the mold, thereby removing the back pressure which causes the melt to drop from the gate upon mold parting. There are instances however where it is impractical or even impossible to use auger decompression to control gate drool. One such instance is in stack molding.
Stack molding is known to provide significant advantages in injection molding of plastic parts, notably among which is increased production output without a corresponding increase in the size of the mold. The higher cavitation and longer melt flow lengths required in hot runner stack molds, however, result in increased pressure losses within the system. These increased pressure losses require increased injection molding pressures to overcome the losses to adequately fill and pack all mold cavities in the mold. Detrimentally, the pressure losses caused by the extensive runner system also results in a decreased ability to achieve satisfactory decompression of the stack mold to prevent drool simply by decompressing the injection auger. Although sufficient decompression may be achieved by releasing auger pressure, the pressure losses in the hot runner system increase the response time of the runner system to the decompression, detrimentally affecting the overall cycle time required for a single molding operation.
Examples of mechanisms for controlling gate drool in single or multi-layered molds are disclosed in U.S. Pat. Nos. 4,473,347 (Terashima) and 5,458,843 (Brown, et al) and Swiss Patent 625,461 (Hotz). These patents disclose various mechanisms for increasing the volume of a melt passage by varying its length or width to achieve a desired decompression. The mechanisms utilize displaceable valve members, expansible pistons or volume increasing cylinder arrangements to create the desired decompression.
There is need for an improved apparatus and method for controlling drool in a mold that is relatively simple in its construction and effective in its operation.
The present invention provides an apparatus and method for achieving increased decompression within a melt passage to impede mold cavity gate drool particularly in thermally gated mold applications.
In one aspect the invention provides a drool control apparatus for a mold, the apparatus comprising:
a melt passage having a first end for receiving a pressurised melt and a second end for communicating with at least one gated nozzle;
a piston disposed in the melt passage; and
means for moving said piston in said melt passage between a bypass position, where said melt is permitted to flow through said melt passage around said piston, and a compression position, where said piston decompresses said melt downstream of said piston to control drool at said at least one gated nozzle.
In another aspect the invention provides a stack injection molding apparatus having a stationary platen and a moving platen, the moving platen moving between an open position and a closed position and cooperating with the stationary platen to define at least one mold cavity when in said closed position, the molding apparatus comprising:
a first runner passage in the stationary platen for supplying a pressurised melt flow to the moving platen;
a second runner passage in the moving platen having a first end in communication with the first runner passage when the moving platen is in the closed position, and a second end in communication with the mold cavity;
a valve unit disposed in the second runner passage, the valve unit having a piston connected thereto; and
an actuator for moving the valve unit in the second runner passage between a bypass position and a compression position, wherein, when the valve unit is in its bypass position, the pressurised melt may flow substantially unimpeded through the second passage to the mold cavity and, when the valve unit is in its compression position, the melt flow through the second passage is at least partially impeded by the piston, whereby the movement of the valve unit from its bypass position to its compression position decompresses the melt downstream of the piston by displacing melt upstream of the piston.
In another aspect the invention provides a stack injection molding apparatus for conducting pressurised melt from a stationary platen to a moving platen moveable between an open position and a closed position comprising:
(a) a first runner passage in the stationary platen communicatively connecting a molding machine inlet to a first gate, and
(b) a melt flow control valve unit disposed in the moving platen, the valve unit having a controllable second gate, a second runner passage extending from the second gate and a piston disposed in the second runner passage, the valve unit being located in the moving platen in a position where in the first and second gates are in conducting communication when the moving platen is in the closed position,
the valve unit being actuatable to open and close the second gate and move the piston within the second runner passage, while the moving platen is in the closed position, to selectively provide a flow of pressurised melt from the first runner passage to the second runner passage and to selectively decompress the pressurised melt in the second runner passage downstream of the piston.
In another aspect of the invention provides a method of controlling mold cavity gate leakage in a molding apparatus when a mold is parted comprising the steps of:
(a) introducing a flow of pressurised melt to a runner passage for subsequent transfer to at least one molding cavity to perform a molding operation,
(b) upon completion of the molding operation, stopping the flow of pressurised melt in the runner passage by moving a piston located within the passage from a bypass chamber, where pressurised melt is permitted to flow around the piston in the passage, to a compression zone, where the passage is at least partially sealed by the piston,
(c) moving the piston upstream in the compression zone to decompress the runner passage downstream of the piston sufficiently to control leakage at said mold cavity gate.